The invention relates to a circuit for supplying a driver circuit for a controllable power-electronic semiconductor switch.
Electronic power switches have two electrodes—usually referred to as the “source” and “drain”—between which the load current to be controlled flows through the semiconductor switch, as well as a control electrode—usually referred to as the “gate”—whose potential difference with respect to that electrode which is usually referred to as the source is decisive for determining whether the electrical connection in the semiconductor switch is open or closed for the load current to be controlled or is in an intermediate state. The relevant reference-ground potential of the gate electrode is thus the potential of the source electrode. If the reference-ground potential of the gate electrode, and the potential of the source electrode, is floating, (i.e., can change with respect to ground depending on the operating state of the circuit) the driver circuit which drives the gate electrode must be accordingly supplied with a voltage related to this floating reference-ground potential.
One example is a transistor half-bridge including two n-channel MOSFETs or IGBTs, as are used, for example, in inverter bridge circuits for driving three-phase motors. A half-bridge is a series circuit including two power semiconductor switches. A “high-side” power switch is connected between a high supply potential and the bridge output node, and a “low-side” power switch is connected between the bridge output node and a low supply potential (e.g., ground). The potential of the bridge output node has a value which is dependent on the switching state of the bridge, (i.e., is floating). The control voltage for the high-side power switch is applied between the gate electrode and the source electrode of the high-side power switch, the source electrode being at the floating potential of the bridge output node. The entire driver circuit which provides the control signals for the high-side power switch must therefore be operated with a voltage supply whose reference-ground potential is the floating potential of the bridge output node.
The voltage supply for the driver circuit, used to drive the gate electrode of the high-side power switch, must therefore be floating with respect to ground. According to the prior art, this is effected either by using a transformer, which is relatively expensive on account of the required inductances, or by using a bootstrap circuit. In this case, a series circuit including a diode and a capacitor is arranged relative to the drain-source path (load path) of the power transistor, the capacitor being connected to the source electrode and the diode being oriented in such a manner that a current can flow through it parallel to the load current through the power transistor. For the driver circuit, the supply voltage is tapped off at the capacitor. If the power transistor is open, the bootstrap circuit is live and the capacitor is charged. If the power switch is closed, (i.e., is on) the capacitor cannot be recharged but forms the voltage supply for the driver circuit. One problem with this design is that the semiconductor switch cannot be kept in an open state for any desired amount of time since eventually the capacitor of the bootstrap circuit will be discharged to such an extent that it no longer suffices as the voltage supply for the driver circuit.
This problem can be circumvented with the aid of additional switches, which are arranged in parallel with the power switch and are operated in clocked fashion, and further bootstrap circuits. However, these circuits are expensive since they each require a plurality of discrete components, (i.e., electronic components) which cannot be accommodated together with other electronic components in an integrated circuit.
For these and other reasons, there is a need for the present invention.